Serge Daan (11 June 1940 – 9 February 2018) was a Dutch scientist, known for his significant contributions to the field of Chronobiology. Early life and education Serge Daan (Mook, 1940) was born in a wind mill, grew up in the Dutch countryside, and went to high school (Gymnasium β) in Deventer. The Daan family was highly interested in biology and undertook enterprises in this field, such as investigating the ecology of reptiles in the Mediterranean area. As a consequence of this interest, Serge studied biology at the University of Amsterdam. Academic career In September 1973, Serge received his Ph.D. in Amsterdam (cum laude) with a thesis on hibernation. He was subsequently trained as a postdoc by the two founders of modern chronobiology, Jürgen Aschoff and Colin Pittendrigh. This 4-year episode, at the Max Planck Institute for Behavioral Physiology in Andechs, Bavaria and Stanford University in California, and the lifelong collaboration and friendship with both were crucial for his professional career. In 1975, Daan was appointed associate professor at the University of Groningen in the Animal Ecology group of R.H. Drent. In 1994 he became Extra-ordinarius in Chronobiology, and in 1996 Professor of Ethology. Since 2003 he has occupied the prestigious Niko Tinbergen chair in Behavioral Biology. Research Serge Daan’s research focuses on the temporal organisation of behaviour in animals and humans. In about 250 publications he contributed a number of key concepts and models that have improved understanding of the ‘circadian’ (circa 24-h) rhythms of rest and activity, the regulation of human sleep, and the annual timing of reproduction. Circadian rhythms Working with Pittendrigh, Daan developed many of the theoretical foundations for understanding the dynamics of circadian oscillators. Many other studies have followed, shifting the focus from behavioural black box models to testable hypotheses about underlying molecular mechanisms. Regulation of human sleep Work on circadian rhythms by others in the field culminated in the notion that a single circadian pacemaker exists to keep track of environmental time, while at the same time controlling downstream oscillations in physiology and behaviour. However, this notion was inconsistent with observations of the timing of sleep in human subjects living in isolation from time cues. Serge Daan, together with Borbély and Beersma, developed a model which convincingly explained the observations. It was called the two-process model of sleep regulation and explained human sleep regulation in terms of two key processes: a circadian pacemaker, and